Hormonal or endocrine treatment for estrogen receptor positive (ER+) breast cancer primarily takes the form of aromatase inhibitors or tamoxifen. Aromatase inhibitors such as Arimidex (anastrozole) and Femara (letrozole) are designed to inhibit the action of the enzyme aromatase, which converts androgens into estrogens within the body. Using aromatase inhibitors to reduce the production of estrogen in the body has been shown to be effective in suppressing breast cancer recurrence, thereby improving outcomes.

Tamoxifen interferes with the body's ability to use estrogen rather than suppressing its production. While the survival benefit of both aromatase inhibitors and tamoxifen is well established, these drugs have side effects that cause a significant fraction of the women who could benefit from them to discontinue use. Now a new study has reported omitting 180 days or more of treatment during the first four years almost triples the risk of breast cancer recurrence.

Aromatase inhibitor and tamoxifen side effects

Taking an aromatase inhibitor results in an extremely estrogen-deficient state which can result in a variety of symptoms. Musculoskeletal problems (such as arthritis and bone fractures) are the most well-known potential side effect of aromatase inhibitors. Some women also experience a range of menopausal symptoms such as hot flashes, hair loss, cold sweats, night sweats, interrupted sleep, leg cramps, breast tenderness, weight gain, cognitive impairment, depression, and/or chest pain. Tamoxifen carries an increased risk of blood clots and endometrial cancer in addition to musculoskeletal problems and menopausal symptoms.

The above lists of side effects are not meant to discourage women who are contemplating or undergoing endocrine treatment, but rather to acknowledge what they are facing. Having said this, it is important to realize that each woman's experience is unique and some will not have overly burdensome symptoms. A switch in aromatase inhibitors can sometimes be very effective in relieving symptoms of women taking an aromatase inhibitor. Women who have problems with Arimidex may do very well on Femara or Aromasin (exemestane). If such a change does not provide enough relief, switching to Tamoxifen could be a good solution. Postmenopausal women on tamoxifen can also try switching to an aromatase inhibitor. Certain foods can also help increase the effectiveness of these treatments or help reduce their side effects.

Latest research finds that recurrence is far more likely if treatment is stopped

The study referenced at the beginning of this news article was designed to investigate the influence of not following prescribed hormonal therapy (i.e., endocrine therapyŚmost commonly, aromatase inhibitors or tamoxifen) on recurrence in women with stage I to stage III breast cancer. To conduct the study, the authors matched women with a breast cancer recurrence within four years of the beginning of hormonal therapy (the "cases") to breast cancer patients on hormonal therapy who did not have a recurrence (the "controls").

A total of 94 cases were matched to 458 recurrence-free survivors. Non-persistence was defined as at least 180 days without hormonal therapy. Non-persistent women were found to have almost three times (2.88 x) the risk of recurrence compared to persistent women. The authors conclude that non-persistence in completing hormonal therapy is associated with a significantly increased risk of early recurrence in women with stage I-III ER+ breast cancer.

Please see our articles on endocrine treatment and survival, breast cancer diet during aromatase inhibitor treatment and what to eat during tamoxifen treatment for more information.